The present invention is related to the subject matter of the following U.S. patents and copending U.S. patent applications:
A. Richard P. Kenan and Carl M. Verber, Electrooptical Multipliers; U.S. Pat. No. 4,403,833, Sept. 13, 1983. PA0 B. Carl M. Verber and Richard P. Kenan, Controlling Light; U.S. Pat. No. 4,415,226, Nov. 15, 1983. PA0 C. Richard P. Kenan and Carl M. Verber, Electrooptical Comparators; U.S. patent application Ser. No. 344,116, filed Jan. 29, 1982. Now U.S. Pat. No. 4,561,728 issued Dec. 31, 1985 PA0 D. Henry John Caulfield, Systolic Array Processing; U.S. patent application Ser. No. 450,153, filed Dec. 15, 1982. Now U.S. Pat. No. 4,567,569, issued Jan. 28, 1986 PA0 E. H. J. Caulfield, Polynomial Evaluation; U.S. patent application Ser. No. 459,168, filed Jan. 19, 1983. Now U.S. Pat. No. 4,544,230, issued Oct. 1, 1985 PA0 F. Carl M. Verber, Optical Computation; U.S. patent application Ser. No. 459,167, filed Jan. 19, 1983. Now U.S. Pat. No. 4,544,229, issued Oct. 1, 1985 PA0 G. Carl M. Verber and Richard P. Kenan, Array Multiplication; U.S. patent application Ser. No. 481,184, filed Apr. 1, 1983, and Ser. No. 573,528, filed Jan. 24, 1984, continuation in part.
Said patents and applications are assigned to the assignee of the present invention. All of the patents and applications cited above are hereby incorporated hereinto by reference and made a part hereof the same as if fully set forth herein for purposes of indicating the background of the present invention and illustrating the state of the art.
Except where otherwise indicated herein, the electrooptic components employed in typical embodiments of the present invention are now well known. Convenient ways of making them are described in the above mentioned patents and applications and in the references cited therein and herein.
The successful demonstration has been reported.sup.1 of a high-speed integrated optic analog to digital (A/D) converter based upon a Mach-Zehnder interferometric technique first suggested by Taylor.sup.2 in 1975. An A/D converter based upon a Fabry-Perot interferometer.sup.3 has been reported also. We present here the design principles, the results of dc tests on a 6-bit device, and dynamic performance estimates of the complementary device, an integrated optic digital to analog (D/A) converter.
The D/A converter is fabricated upon a planar single-mode Ti-indiffused LiNbO.sub.3 waveguide. The active element is an electrooptic integrated optic spatial light modulator (IOSLM),.sup.4 which is simply an extended interdigital electrode structure composed of a number of separately addressable segments. The electrode segments are addressed, in parallel, with the voltages representing the digital word to be converted.
In the configuration tested, it is essential that a digital zero be represented by a zero voltage and that all digital ones be represented by a voltage V. As shown in FIG. 1, the voltages representing the digital word are applied to the electrodes through voltage dividers. The dividers are set so that the voltage V, when representing the most significant bit, results in the diffraction of an optical power which we may represent by P.sub.max. The next divider is set so that the diffracted power is P.sub.max /2, the next to generate P.sub.max /4, and so on. The total optical power diffracted by the structure is, therefore, the optical analog representation of the electrical digital input. This optical analog signal may then be used as the input to an analog optical device such as a multiplier,.sup.4 or a lens can be used to direct all of the diffracted light to a photodetector, in which case the electrical analog signal is generated.
FIG. 2 shows the results of a simple proof-of-principle experiment, which was set up by uniformly illuminating the IOSLM with a prism-coupled guided plane wave. The diffracted light was collected by an external lens and directed onto a photodetector. The voltage dividers were individually set as described above, and the system was stepped manually through the digital words 000000 to 111111 by use of toggle switches. The figure shows the analog voltage generated by the photodetector as a function of the digital input word. As can be seen, the system functioned as expected. The kink in the otherwise straight line is thought to be due to a slight missetting of one of the voltage dividers.
The high-speed performance of the integrated optic D/A can be estimated by assuming, for example, that a laser will be used which will result in a diffracted power of 50 .mu.W from the most significant bit. In this case the maximum diffracted power, when all 6 bits are on, will be 98.44 .mu.W, and the contribution of the least significant bit (LSB) will be 1.56 .mu.W, a value which is -36 dB down from the maximum. It can be shown that, for direct detection of a 100 .mu.W signal at a 100-MHz bandwidth, the signal to noise ratio (SNR) of an optical detector is 60 dB..sup.5 Therefore, the LSB can be detected with an excess SNR of 24 dB. This excess can be retained to achieve a minimum error rate, to increase the number of bits, to increase the operating rate, or to decrease the optical power.